1. Field of the Invention
Many fluid processes operate by passing fluids through beds of material. These processes include chromatography, ion exchange, adsorption, catalytic reaction, etc. This invention is directed to these processes in general.
2. State of the Art
Fluid processes characteristically exhibit severe limits on operation due to bed pressure drop, kinetics and flow uniformity. These limits are placed on, for example, productivity, process efficiency, energy use, system size, environmental compatibility, and capital/operating costs.
As one example of how these limits occur, the flow rate through a bed may be constrained because as flow rate increases, bed pressure drop increases. Pressure drop may reach a point where the pressure rating of a column containing the bed may be exceeded, the bed may begin to unacceptably compress, bed particles may be destroyed and excessive energy may be required for operation. Clearly, this effect places limits on productivity (limits on flow rate) and cell design and cost (higher pressure requires additional structural strength).
As another example, high linear velocities can result in unacceptably poor interaction or reaction of a fluid with the bed material. That is, the kinetic requirements of the system are self limiting. An excessively high linear velocity of a fluid through a bed will result in an insufficient contact time of the fluid with the bed particles. Clearly this places limits on productivity. (Again, flow rate is limited).
Spreading out a bed to a wide (large cross section), shallow (shallow depth or short travel path) geometry instead of a high (long travel path), narrow (relatively small cross section transverse to the direction of flow) geometry will reduce both the bed pressure drop and the linear velocity of a fluid passing through the bed. While both of these effects would be very beneficial, such column construction is not prevalent because of the difficulty of distributing and collecting fluid across a wide, shallow bed (a large cross section). Any inhomogeneity or turbulence in the fluid introduced into the column cannot normally be attenuated through a wide, shallow bed so the inhomogeneities are reflected as inefficiencies and unacceptable processing. For example, in chromatography, such problems result in band broadening and poor separation of the components of a feed mixture.
A representative device is disclosed in U.S. Pat. No. 4,673,507 to Brown. The '507 patent discloses a fluid treatment apparatus which can be used for shallow bed operation. However this device lacks significantly distributed fluid feed and collection systems and is dependent upon maintaining the bed in an over packed condition. A substantially uniform fluid flow distribution across the bed is achieved by employing resins of fine (substantially uniform) particle size which are maintained in an over packed condition. Here, the term ‘over packed’ is used to mean that the particles are confined within the resin bed so that they are subjected to compression at all times. This device inherently restricts process fluid flow across the bed.
U.S. Pat. No. 5,626,750 to Chinn discloses an apparatus for treating fluid. In this apparatus, first and second “particle free cavities” are provided above and below the retained particle bed. Even flow of fluid through the bed is provided simply by the pressure drop across the bed. The pressure drop across the bed is a function of the pressures in the first and second cavities. No provision is made to substantially control fluid flow characteristics (eddies, or turbulent zones) in process fluid streams near the bed surface.
It would be an advance to provide an apparatus for processing fluids which exhibits a reduced pressure drop through a bed of media, and also has a reduced fluid flow rate (velocity) at an increased volumetric flow rate through the bed. A preferred apparatus would provide control over process fluid flow to reduce mixing and turbulence near the bed to resist inhomogeneities in the processing stream.